1. Field of the Invention
The invention relates generally to the field of communications. More particularly, the invention relates to methods of multicarrier orthogonal spread-spectrum communications, and machinery for transmitting and/or receiving such communications.
2. Discussion of the Related Art
In digital data-transmission systems, the principal goal is generally to maximize the data-transmission rate, consistent with the legally permissible occupied RF (radio frequency) bandwidth for the signal and the available signal-to-noise ratio (SNR) for the selected channel or RF link path. An additional consideration is the maximum transmitted RF power levels allowed in the various bands. For instance, in the 902-928 MHz ISM (Industrial, Scientific, and Medical) band, up to 1 watt of RF power is currently permitted in the U.S., with a maximum isotropic antenna power gain of 4 (6 dBi); for higher antenna gains, the RF power must be reduced linearly to provide no more than 4 W EIRP (effective isotropic radiated power). In the 2.450-2.4835 GHz band, ¼ watt of power can currently be used, with the same maximum antenna gain of 6 dBi, for a maximum nominal EIRP of 1 W; for higher antenna gains, the RF power must be reduced by 1 dB for every 3 dB of additional antenna gain. In the 5.725-5.825 GHz ISM band (which at present experiences the least RF interference of the three ISM bands and is thus preferred for data-link applications) for FCC-compliant spread-spectrum devices, up to 1 W of RF output power is currently permitted, without regard for antenna gains of greater than 6 dBi, so long as the minimum direct-sequence process gain of 10 dB is maintained. OFDM systems in the U.S. fall under specific Unlicensed National Information Infrastructure (U-NII) as well as ISM regulations. OFDM systems are presently not permitted at all in the 915-MHz ISM band and are limited to a ¼-watt RF output power level in the 5.8-GHz ISM band (and much lower levels in the 5.1- and 5.3-GHz U-NII bands).